1. Field of the Invention
The present invention relates to a method for bleaching teeth for removing pigments deposited on teeth (coloration and discoloration of teeth) by an action of a photocatalyst, and a bleaching agent for teeth suitable for carrying out the method for bleaching teeth. More specifically, it relates to a method for bleaching teeth by applying a bleaching agent for teeth having a photocatalytic activity on a surface of teeth, and irradiating the applied part with light to bleach the teeth based on a photocatalytic action thus produced, and a bleaching agent for teeth useful for carrying out the method for bleaching teeth comprising a solution containing nitrogen-deeped titanium oxide powder that produces a photocatalytic action upon irradiation with light.
2. Description of Conventional Art
It is generally considered that whiteness of teeth is an important factor of beautification, and there are strong demands for whitening teeth centrally in young women to produce increasing cases of desiring bleach of teeth. As a method for bleaching teeth, a method using a hydrogen peroxide aqueous (H2O2) solution has been generally practiced.
That is, such a bleaching method has been generally practiced as a method for bleaching teeth in that light and heat are applied to a hydrogen peroxide aqueous solution (concentration: about 30% by weight), in which gauze impregnated with a hydrogen peroxide aqueous solution is placed on a labial surface of teeth and irradiated with light by lamps from side to side for about 30 minutes. In this method, the lamps are made close to the teeth as much as possible, and a hydrogen peroxide aqueous solution is supplied by about 5 minutes to prevent the gauze from drying.
There are also a method of repeating such an operation six to eight times instead of the irradiation with light that a high frequency electric current is applied for 1 second with a spoon-shaped chip equipped on a high frequency electric cautery knife, and then the operation is suspended for 8 seconds, and a method of directly applying a solution (paste) formed by mixing a thickener with a hydrogen peroxide aqueous solution to teeth instead of impregnation into gauze. However, a hydrogen peroxide aqueous solution having a concentration exceeding 25% by weight is necessarily handled carefully due to the strong corrosion nature thereof.
Many other bleaching agents and bleaching methods using a hydrogen peroxide aqueous solution having a concentration of 30 to 35% combined with other equipments and other agents have been proposed, such as a bleaching method of using a mixed solution of hydrochloric acid, a hydrogen peroxide aqueous solution and diethyl ether as an agent (modified McInnes bleaching method), a method of using a paste formed by kneading powder of sodium perborate and a 30% by weight hydrogen peroxide aqueous solution as an agent (working bleach method), a bleaching agent for teeth formed by mixing a hydrogen peroxide aqueous solution and orthophosphoric acid and a bleaching method using the same (JP-A-8-143436), a bleaching agent formed by mixing a hydrogen peroxide aqueous solution and silicic anhydride and a method for bleaching vital teeth of coating the bleaching agent (JP-A-5-320033), and a dental bleaching composition containing a dental bleaching agent (such as urea hydrogen peroxide, hydrogen carbamide peroxide and carbamide peroxide) and a matrix material (such as carboxymethylene) and a method for bleaching teeth using the same (JP-A-8-113520). However, these methods involve the same problems as in the foregoing methods from the standpoint of the use of hydrogen peroxide in a high concentration. There is also a bleaching method using urea peroxide in a concentration of about 10% by weight, instead of a hydrogen peroxide aqueous solution, as an example of bleaching methods that have been practiced in the United States, but no sufficient results have been obtained.
As a bleaching agent and a bleaching method for teeth using no hydrogen peroxide aqueous solution in a high concentration as described in the foregoing, such a method for bleaching teeth is also proposed that uses titanium dioxide having a photocatalytic action and, depending on necessity, a hydrogen peroxide aqueous solution. However, the conventional bleaching method and bleaching agent using titanium dioxide has such a defect that they exhibit substantially no catalytic action with respect to visible light while they exhibit catalytic action with respect to ultraviolet light (generally having a wavelength of less than 380 nm) owing to the band gap of the titanium dioxide used (Eg for the anatase type titanium dioxide is 3.2 eV). Ultraviolet light is malefic to a human body and thus is not suitable for long-term irradiation in an oral cavity, and therefore, it has been demanded to develop a novel method for bleaching teeth and a novel bleaching agent for teeth using a substance that effectively exhibits a photocatalytic action by using a light source containing substantially no ultraviolet light, such as a dental light source.